'70's Cyclone Purple Hornies

I was obsessed as a kid with the way cars sounded. My earliest recollection was "smitty's" on a '53 chevy custom car. After that I was hooked. In the '70's it was "Purple Hornies" and their ad stating,"with the race-car bark". My question is How were they built and why did they sound so nice (and loud)? I tried to look up into one under a friends Chevelle and couldn't quite see in there so I put my hand inside of it and it felt like ribs of rubber running parallel to the muffler. Anyone out there ever dissect one of these? The new ones today don't sound anything like the old ones.